Hmm, now let’s see here… According to this helpful graphic on the back of an old DC Digest…
When Superboy grows up, he becomes Superman. Sure, sure, that makes sense.
When Lex Luthor grows up, he, well, sure — he’s still Lex Luthor.
And when Supergirl grows up…
She trades her skirt for hot pants. Got it.
(From Best of DC Blue Ribbon Digest #7, October 1980.)